The present invention relates to a charge-pump circuit used for a power supply circuit, and relates in particular to a charge-pump circuit that can freely adjust a boosting voltage at a step whereat a voltage is smaller than a power voltage, and that can provide circuit efficiency that is greatly improved.
A charge-pump circuit developed by Digson is so designed that pumping packets are connected in series at multiple stages, and a voltage higher than the power source Vdd of an LSI chip is generated due to the fluctuation of the voltage in each pumping packet. This charge-pump circuit is used, for example, to generate a voltage for programming/erasing flash memories.
This example charge-pump circuit employs an output load current of several tens of xcexcA. This low load current charge-pump circuit is effective because all the MOSFETs that serve as coupling capacitors or diodes can be incorporated in a single LSI. Recently, as the voltage used for flash memories has been lowered, a charge-pump circuit has been proposed that generates a high voltage and that provides improved boosting efficiency.
Aside from this, the portable devices, such as video cameras, digital cameras and portable telephones, that have recently become popular require high voltages and large currents (several mA) for liquid crystal displays, and for these devices, switching regulators are used as voltage generators.
In accordance with the principle on which a switching regulator is based, a large current flows to a coil while a counterelectromotive force raises the voltage precipitously. The switching regulator is characterized by its efficient production of a high voltage and a large current, and thus it is an appropriate means for providing the power efficiency (output power/input power) that is especially important for portable devices. However, when a large current flows across the coil, harmonic noise is generated, and shielding is required for the power supply circuit in order to prevent the harmonic noise from having an adverse affect. For a portable device, the generation of a high voltage with little attendant noise is desirable because of the reduced size of the unit and the increased sensitivity this entails.
When a voltage loss, such as the threshold value for a diode, is ignored, theoretically, at each stage the pumping packets of the charge-pump circuit receive a boost equivalent to the power voltage Vdd. So that when the input voltage Vin of the charge-pump circuit is defined as Vdd and the pumping packets that are arranged at n stages are connected in series to the power source circuit, the voltage can be boosted by (n+1)xc3x97(Vddxe2x88x92Vt), wherein Vdd denotes the power voltage, and Vt denotes the threshold voltage of a diode in the forward direction. To simplify the following explanation, Vt=0V.
FIG. 8 is a schematic circuit diagram showing a conventional charge-pump circuit (n=4). In FIG. 8, diodes D1 to D5 are connected in series, and capacitors C1 to C4 are each connected at one end to a junction of two of the diodes D1 to D5 and at the other end to a clock driver 1 that supplies clocks "PHgr"1 and "PHgr"2, which have reverse phase clock pulses, to the capacitors C1 to C4. A current load 2 is driven by the boosting voltage VH and the output current Iout that are output by the diode D5. And the charge-pump circuit has four pumping packets and corresponds to a charge-pump circuit having a four-stage arrangement.
The operation of this charge-pump circuit will now be described. When the clock "PHgr"1 at level L and the clock "PHgr"2 at level H are output by the clock driver 1, the current 2xc3x97Iout flows in the direction indicated by solid-line arrows and Iout denotes the current output at the final stage by the diode D5.
Then, when the clock "PHgr"1=level H and the clock "PHgr"2=level the current 2xc3x97Iout flows in the direction indicated by broken-line arrows. Since these are alternately flowing currents, voltage boosting is performed at the individual stages, until at the final stage, a boosted voltage VH of 5 Vdd is output by the diode D5.
Supposing that the current output from the diode D5 at the final stage is Iout, on the average, the current input to the diode D1 at the first stage is equal to Iout as are the currents flowing in the directions indicated by the solid-line arrows and the broken-line arrows. Therefore, when the efficiency xcex7 of the charge-pump circuit is defined as xcex7=output power/input power (%), the efficiency xcex7 of the circuit is represented as follows.
xcex7=5 Vddxc3x97Iout/Vddxc3x975 Iout=100%
That is, under the operating conditions described above, the efficiency xcex7 of the charge-pump circuit is 100%.
As is described above, the charge-pump circuit outputs the boosted voltage (n+1)xc3x97Vdd, wherein n denotes the number of pumping steps in the charge-pump circuit and Vdd denotes a power voltage. Therefore, when Vdd=5V, the theoretical value VHn of the output voltage of a charge pump circuit having n steps is the step voltage for the Vdd step, i.e., when n=1, VH1=2xc3x97Vdd=10V; when n=2, VH2=3xc3x97Vdd=15V; and when n=3, VH3=4xc3x97Vdd=20V.
When the charge-pump circuit is employed as a high voltage generator, a voltage may be adjusted by a regulator in order to set a desired high voltage. FIG. 9 is a schematic circuit diagram showing an n-step charge-pump circuit that includes a regulator. The same reference numerals as are used in FIG. 8 are used to denote corresponding or identical components in FIG. 9, and no further explanation for them will be given.
In FIG. 9, diodes D1 to Dn are connected in series. A boosted voltage VHn output at the final stage by the diode Dn is dropped by a regulator 3, and the obtained voltage is supplied to a current load 2. When the final output voltage obtained via the regulator 3 is defined as Vout and the output current is defined as lout, the efficiency xcex7 of the charge pump circuit is defined as
xcex7=output power/input power=Voutxc3x97Iout/(n+1)xc3x97Vddxc3x97Iout=Vout/(n+1)Vdd.
When the number n of charge pump steps is large, i.e., the voltage boosting ratio=VHn/Vdd is large, the charge-pump circuit can attain a substantially high efficiency. But when the power voltage fluctuation range is wide and Vout/Vdd is small, efficiency is deteriorated. Assume, then, that the number n of charge pump steps is variable and that Vdd=4 to 5.5V and Vout=6.5V. For this assumption, the step number n=1 is optimal, and for the charge-pump circuit, the efficiency ratios xcex7 provided by Vdd=4V, 5V and 5.5V are as follows.
As is apparent, an increase in the power voltage Vdd is accompanied by a deterioration in the efficiency ratio xcex7. But when n=0.5, the following high efficiency ratios xcex7 can be provided.
This means that a charge-pump circuit can use 0.5 Vdd steps to boost voltage. However, conventional charge-pump circuits use Vdd steps to boost voltage, and up to the present no charge-pump circuit has been proposed that can use steps smaller than Vdd to boost voltage.
It is, therefore, one object of the present invention to provide a charge-pump circuit that can use a voltage step smaller than Vdd to boost voltage, and that can attain an improved circuit efficiency ratio xcex7. Specifically, since the charge-pump circuit proposed by this inventor can theoretically provide output voltages of 1.5 Vdd, 2 Vdd, 2.5 Vdd, 3Vdd, . . . , it is another object of the present invention to expand this basic charge-pump circuit to boost voltages at smaller voltage steps, such as 1.5 Vdd, 1.75 Vdd, 2 Vdd, 2.25 Vdd, 2.5 Vdd, 2.75 Vdd, 3 Vdd, . . . , and it is an additional object of the present invention to boost voltages at arbitrary voltage steps, such as 1.33 Vdd, 1.66 Vdd, 2 Vdd, 2.33 Vdd, 2.66 Vdd and 3 Vdd, and to provide negative voltages as well. These and other objectives of the invention, as well as additional innovative features, will become obvious during the course of the description of the specification and when the accompanying drawings are referred to.
An overview of the aspects of the invention will now be described as follows. According to a first aspect of the invention, a charge-pump circuit comprises: a plurality of diodes connected in series; at least two capacitors connected to junctions of the diodes; clock supply means for supplying a clock to the capacitors; and switching means for connecting the capacitors in series to the junctions of the diodes and for connecting the capacitors in parallel to the junctions of the diodes in accordance with a voltage level of the clock, wherein a boosted voltage is output by the diodes.
This arrangement is the essential configuration of the invention, and it can provide a voltage boost while employing a voltage step that is smaller than a power voltage, while improving the efficiency ratio xcex7 of the circuit.
According to a second aspect of the invention, a charge-pump circuit comprises: a plurality of diodes connected in series; a power source for supplying a power voltage to a diode at a first stage; at least two capacitors connected to junctures of the diodes at the first stage and at a second stage; clock supply means for supplying a clock to the capacitors; and switching means for connecting the capacitors in series to the junctures of the diodes when the clock is at level L, and for connecting the capacitors in parallel to the junctures of the diodes when the clock is at level H, wherein a positive, boosted voltage is output by a diode at the final stage at a step whereat the voltage is smaller than the power voltage.
With this arrangement, a voltage can be boosted at a step whereat the positive voltage is smaller than the power voltage, and for the circuit, the efficiency ratio xcex7 can be improved.
According to a third aspect of the invention, a charge-pump circuit comprises: a plurality of diodes connected in series; a power source for supplying a power voltage to a diode at a first stage; at least two, first capacitors connected to junctures between the diodes at the first stage and at a second stage; at least one, second capacitor connected to junctures of the remaining diodes; clock supply means for alternately supplying clocks in reverse phases to the first capacitors and the second capacitor; and switching means for changing the connections of the first capacitors in accordance with the voltage level of the clock supplied to the first capacitors, wherein when the clock supplied to the first capacitors is at level L, the switching means connects the first capacitors in series to a juncture between diodes at a first stage and a second stage, wherein, when the clock supplied to the first capacitors is at level H, the switching means connects the first capacitors in parallel to the juncture, and wherein a diode at the final stage outputs a positive, boosted voltage obtained by adding a step whereat the voltage is the same as the power voltage to a step whereat the voltage smaller than the power voltage.
With this arrangement, a step whereat the voltage is smaller than the power voltage can be added to the step whereat the voltage is the same as the power voltage, and the thus obtained positive, boosted voltage can be output. Further, for the circuit, the efficiency ratio xcex7 can be improved.
According to a fourth aspect of the invention, the charge-pump circuit further comprises switch control means for controlling the switching means, so that, in response to a switch control signal output by the switch control means, the two or more capacitors are always connected in series to the juncture between the diodes at the first stage and the second stage, or are connected in series or in parallel in accordance with the level of the clock.
According to this arrangement, voltage boosting at the step whereat the voltage equals the power voltage, and voltage boosting at the step whereat the voltage is smaller than the power voltage can be implemented by a single charge-pump circuit. Further, the boosted voltage can be set more accurately, and for the circuit, the efficiency ratio xcex7 can be improved.
According to a fifth aspect of the invention, the charge-pump circuit of further comprises: a regulator for adjusting a boosted voltage output by the diode at the final stage.
According to a sixth aspect of the invention, the charge-pump further comprises: voltage detection means for detecting the boosted voltage output by the diode at the final stage; and charge-pump circuit step count control means for controlling the number of steps in the charge-pump circuit in accordance with the detection results that are obtained.
With this arrangement, the number of steps in the charge-pump circuit can be changed in accordance with the boosted voltage, and for the circuit, the efficiency ratio xcex7 can be further improved by adjusting the boosted voltage.
According to a seventh aspect of the invention, in the charge-pump circuit, the diodes are constituted by a MOS transistor wherein a gate and a source are connected in common. With this arrangement, diodes need not be separately formed during the MOS processing, and thus are easily manufactured.
According to an eighth aspect of the invention, in the charge-pump circuit, the switching means is constituted by a MOS transistor. According to this arrangement, since only a small number of circuit devices are required the structure is simple.
According to a ninth aspect of the invention, a charge-pump circuit comprises: a plurality of diodes being connected in series, with a ground voltage being applied to a diode at a first stage; at least two capacitors connected to junctures of said diodes at said first stage and at a second stage; clock driver means for supplying a clock to said capacitors; and switching means for connecting said capacitors in series to said junctures of said diodes when said clock is at level H, and for connecting said capacitors in parallel to said junctures of said diodes when said clock is at level L, wherein a negative, boosted voltage is output by a diode at the final stage at a step whereat the voltage is smaller than a power voltage supplied to said clock driver means.
Furthermore, according to the tenth aspect of the invention, a charge-pump circuit comprises: a plurality of diodes being connected in series, with a ground voltage being applied to a diode at a first stage; at least two, first capacitors connected to junctures between said diodes at said first stage and at a second stage; at least one, second capacitor connected to junctures of the remaining diodes; clock supply means for alternately supplying clocks in reverse phases to said first capacitors and said second capacitor; and switching means for changing the connections of said first capacitors in accordance with the voltage level of said clock supplied to said first capacitors, wherein a negative, boosted voltage is output by a diode at the final stage at a step whereat the voltage is smaller than a power voltage supplied to said clock driver means.